If the other suits are drones, then maybe they don't require arc reactors to work. Or maybe Stark can spread out (and dilute) the power over multiple drone armors.

And maybe Tony is using the Iron Legion as a diversion....something to keep AIM and the Extremis Soldiers occupied, while he concentrates on singling out Mandarin and/or Killian for the inevitable mano-a-mano duel. That would explain why he brought Hulkbuster along --- and everything else but the kitchen sink. (I don't think Stealth armor, for instance, is going to be useful here, since a full-on frontal assault is about as UN-stealthy as it gets; but it's clear that Tony brings *everything* in his closet to the party.)

Would be freakin' awesome if he actually refers to it in the movie as the Hulkbuster.